Wireless communication systems are widely known in which base stations (BSs) communicate with user equipments (UEs) (also called terminals, or subscriber or mobile stations) within range of the BSs.
The geographical area covered by one or more base stations is generally referred to as a cell, and typically many BSs are provided in appropriate locations so as to form a network covering a wide geographical area more or less seamlessly with adjacent and/or overlapping cells. (In this specification, the terms “system” and “network” are used synonymously). Each BS divides its available bandwidth into individual resource allocations for the user equipments which it serves. The user equipments are generally mobile and therefore may move among the cells, prompting a need for handovers between the base stations of adjacent cells. A user equipment may be in range of (i.e. able to detect signals from) several cells at the same time, and it is possible for one cell to be wholly contained within a larger cell, but in the simplest case the UE communicates with one “serving” cell.
The direction of communication from the base station to the UE is referred to as the downlink (DL), and that from the UE to the base station as the uplink (UL). Two well-known transmission modes for a wireless communication system are TDD (Time Division Duplexing), in which downlink and uplink transmissions occur on the same carrier frequency and are separated in time, and FDD (Frequency Division Duplexing) in which transmission occurs simultaneously on DL and UL using different carrier frequencies.
Resources in such a system have both a time dimension and a frequency dimension. In LTE, the time dimension has units of a symbol time or “slot” (where a “slot” has typically a duration of seven symbol times), as indicated in FIG. 1. The resources in the time domain are further organised in units of frames, each having a plurality of “subframes”. Frames follow successively one immediately after the other, and each is given a system frame number (SFN).
In one frame structure for LTE, the 10 ms frame is divided into 20 equally sized slots of 0.5 ms as illustrated in FIG. 1. A sub-frame consists of two consecutive slots, so one radio frame contains 10 sub-frames. An FDD frame consists of 10 uplink subframes and 10 downlink subframes occurring simultaneously. In TDD, the 10 subframes are shared between UL and DL and various allocations of subframes to downlink and uplink are possible, depending on the load conditions. Subframes may consequently be referred to as uplink subframes or downlink subframes.
Meanwhile the frequency dimension is divided in units of subcarriers. The UEs are allocated, by a scheduling function at the BS, a specific number of subcarriers for a predetermined amount of time. Such allocations typically apply to each subframe. Resources are allocated to UEs both for downlink and uplink transmission (i.e. for both downlink subframes and uplink subframes).
The transmitted signal in each slot is described by a resource grid of sub-carriers and available OFDM (Orthogonal Frequency-Division Multiplexing) symbols, as shown in FIG. 2. Each element in the resource grid is called a resource element, and each resource element corresponds to one symbol. Each downlink slot has a duration Tslot with either 7 or 6 symbols per slot, depending on whether a short or long cyclic prefix (CP) is used. There are a total of NBW subcarriers in the frequency domain, the value of this number depending on the system bandwidth. A block of 12 subcarriers×7 or 6 symbols is called a Resource Block. The Resource Block is the basic unit of scheduling for allocation of resources in the UEs.
A base station typically has multiple antennas and consequently can transmit (or receive) multiple streams of data simultaneously. Physical antennas controlled by the same base station may be widely geographically separated, but need not be so. A group of physical antennas which provides a logically distinct communication path to a UE is termed an antenna port (and may also be considered to be a virtual antenna). Antenna ports may comprise any number of physical antennas. Various transmission modes are possible via the antenna ports, including (in LTE-A) a “transmission mode 9” for closed-loop multiple-input, multiple-output (MIMO). A subset of the physical antennas, which are all in the same geographical location, may be regarded as a distinct transmission point (TP) under control of the same base station. There is generally a fixed relationship between the TPs and cells: each TP may define a distinct cell in the network, but need not do so. References in the remainder of this specification to “base station” apply also to an individual TP unless the context demands otherwise.
Several “channels” for data and signalling are defined at various levels of abstraction within the network. FIG. 3 shows some of the channels defined in LTE at each of a logical level, transport layer level and physical layer level, and the mappings between them.
At the physical layer level, on the downlink, user data as well as System Information Blocks (SIBs) are contained in a transport channel DL-SCH, carried on the Physical Downlink Shared Channel (PDSCH). As can be seen from FIG. 3, PDSCH also carries a paging channel PCH at the transport layer level. There are various control channels on the downlink, which carry signalling for various purposes; in particular the Physical Downlink Control Channel, PDCCH, is used to carry, for example, scheduling information from a base station (called eNodeB in LTE) to individual UEs being served by that base station. The PDCCH is located in the first OFDM symbols of a slot.
Each base station broadcasts a number of channels and signals to all UEs within range, whether or not the UE is currently being served by that cell. Of particular interest for present purposes, these include a Physical Broadcast Channel PBCH as shown in FIG. 3, as well as (not shown) a Primary Synchronization Signal PSS and Secondary Synchronization Signal SSS, described in more detail below. PBCH carries a so-called Master Information Block (MIB), which gives, to any UEs within range of the signal, basic information including system bandwidth, number of transmit antenna ports, and system frame number. Reading the MIB enables the UE to receive and decode the SIBs referred to earlier.
Meanwhile, on the uplink, user data and also some signalling data is carried on the Physical Uplink Shared Channel (PUSCH), and control channels include a Physical Uplink Control Channel, PUCCH, used to carry signalling from UEs including channel quality indication (CQI) reports and scheduling requests.
The above “channels” defined for various data and signalling purposes, should not be confused with the “channel” in the sense of the radio link between a UE and its serving base station(s), which is subject to fading and interference. To facilitate measurements of the channel by UEs, the base station inserts reference signals in the resource blocks as shown, for example, in FIG. 4. FIG. 4 shows the downlink reference signal structure for single antenna port transmission. As can be seen, one subframe has reference signals, denoted R, inserted at intervals within individual REs. Various kinds of reference signal are possible, and the reference signal structure or pattern varies when more antenna ports are in use.
In LTE (as distinct from LTE-A), downlink reference signals can be classified into a cell-specific (or common) reference signal (CRS), an MBSFN reference signal used in MBMS (not relevant for present purposes), and user equipment-specific reference signals (UE-specific RS, also referred to as demodulation reference signals, DM-RS). There is also a positioning reference signal.
The CRS is transmitted to all the UEs within a cell and used for channel estimation. The reference signal sequence carries the cell identity. Cell-specific frequency shifts are applied when mapping the reference signal sequence to the subcarriers. A UE-specific reference signal is received by a specific UE or a specific UE group within a cell. UE-specific reference signals are chiefly used by a specific UE or a specific UE group for the purpose of data demodulation.
CRSs are transmitted in all downlink subframes in a cell supporting non-MBSFN transmission, and can be accessed by all the UEs within the cell covered by the eNodeB, regardless of the specific time/frequency resource allocated to the UEs. They are used by UEs to measure properties of the radio channel—so-called channel state information or CSI. Meanwhile, DM-RSs are transmitted by the eNodeB only within certain resource blocks that only a subset of UEs in the cell are allocated to receive.
Starting with Release 10 of the specifications, LTE is referred to as LTE-Advanced (LTE-A). A new reference signal in LTE-A is a Channel State Information Reference Signal (CSI-RS). To minimise interference, CSI-RS is only transmitted once every several subframes. In the Release 10 specifications, configurations of CSI-RS patterns are defined for 1, 2, 4 or 8 antenna ports. Their purpose is to allow improved estimation of the channel for more than one cell for feeding back channel quality information and possibly other related parameters to the network (compared with using CRS). CSI-RS patterns in time and frequency can be configured by higher layers to allow considerable flexibility over which resource elements (REs) contain them.
A UE compliant with LTE Release 10 can be configured with multiple CSI-RS patterns specific to its serving cell:                one configuration for which the UE shall assume non-zero transmission power for the CSI-RS; and        zero or more configurations for which the UE shall assume zero transmission power.        
The purpose of the ‘zero power CSI-RS patterns’ is to ensure that a cell so configured can safely be assumed by the UE to not transmit in the REs which will contain CSI-RS of the cells it is cooperating with. Knowledge of the presence of zero power CSI-RS patterns can be used by a Release 10 UE to mitigate their possible impact on data transmissions using PDSCH.
Reference signals are also defined on the uplink, in particular a Sounding Reference Signal (SRS) transmitted by the UE, which provides channel information to the eNodeB.
A UE receives two SRS configurations from the network via RRC signalling. One is a UE-specific SRS configuration which details periodicity, offset, transmission comb index, frequency domain position and frequency hopping pattern of SRS transmissions. SRS are always transmitted in the last OFDM symbol of a subframe where they occur. There is also a cell-specific SRS configuration to indicate to all UEs when and where SRS may occur so that the UEs can stop PUSCH transmissions in all relevant frequency and time domain resources. In LTE Rel-10, SRS may be periodic according to the configuration or aperiodic (triggered by the network via DL signalling).
A UE must successfully perform a cell search procedure and obtain synchronization with a cell before communicating with the network. Each cell is identified by a physical layer cell identity (PCI), 504 of which are defined in LTE. These are arranged hierarchically in 168 unique cell layer identity groups each containing three physical layer identities. To carry the physical layer identity and the physical layer cell identity group, two signals are provided: the primary and secondary synchronization signals (PSS and SSS). Specified in 3GPP TS36.211, hereby incorporated by reference, the PSS specifies one of three values (0, 1, 2) to identify the cell's physical layer identity, and the SSS identifies which one of the 168 groups the cell belongs to. In this way it is only necessary for PSS to express one of three values whilst SSS expresses one of 168 values. PSS is a 62-bit signal based on a Zadoff-Chu sequence, and SSS uses a combination of two 31-bit sequences which are scrambled by use of a sequence derived from the physical cell identity. Both PSS and SSS are transmitted in fixed resources by all cells so that they can be detected by any UE within range of the signal. Conventionally, each of the PSS and SSS is transmitted twice per frame, in other words with a 5 ms periodicity (and consequently, only in some subframes). For example, PSS and SSS are both transmitted on the first and sixth subframe of every frame as shown in FIGS. 5A and 5B. FIG. 5A shows the structure of PSS AND SSS and PBCH in the case of an FDD system (using a normal CP), and FIG. 5B shows the same thing in the case of TDD.
Successfully decoding the PSS and SSS allows a UE to obtain timing and identity for a cell. Once a UE has decoded a cell's PSS and SSS it is aware of the cell's existence and may decode the MIB in the PBCH referred to earlier. Depending on whether the system is using FDD or TDD, PBCH occupies the slots following or preceding PSS and SSS in the first subframe, as can be seen by comparing FIG. 5A and FIG. 5B. Like the synchronization signal SSS, PBCH is scrambled using a sequence based on the cell identity. The PBCH is transmitted every frame, thereby conveying the MIB over four frames.
The UE will then wish to measure the cell's reference signals (RSs). For current LTE releases, the first step is to locate the common reference signals CRS, the location in the frequency domain of which depends on the PCI. Then the UE can decode the broadcast channel (PBCH). In addition, the UE can decode PDCCH and receive control signalling. In particular, in the case of Transmission Mode 9, the UE may need to measure the radio channel using the Channel State Information RS (CSI-RS) mentioned above.
Having synchronized with the network and decoded the MIB, UE will also need to obtain some uplink transmission resource for sending its data to the network.
The Physical Random Access Channel PRACH is used to carry the Random Access Channel (RACH) for accessing the network if the UE does not have any allocated uplink transmission resource. If a scheduling request (SR) is triggered at the UE, for example by arrival of data for transmission on PUSCH, when no PUSCH resources have been allocated to the UE, the SR is transmitted on a dedicated resource for this purpose. If no such resources have been allocated to the UE, the RACH procedure is initiated. The transmission of SR is effectively a request for uplink radio resource on the PUSCH for data transmission.
Thus, RACH is provided to enable UEs to transmit signals in the uplink without having any dedicated resources available, such that more than one terminal can transmit in the same PRACH resources simultaneously. The term “Random Access” (RA) is used because (except in the case of contention-free RACH, described below) the identity of the UE (or UEs) using the resources at any given time is not known in advance by the network (incidentally, in this specification the terms “system” and “network” are used interchangeably). Preambles (which when transmitted, produce a signal with a signatures which can be identified by the eNodeB) are employed by the UEs to allow the eNodeB to distinguish between different sources of transmission.
RACH can be used by the UEs in either of contention-based and contention-free modes.
In contention-based RA, UEs select any preamble at random, at the risk of “collision” at the eNodeB if two or more UEs accidentally select the same preamble. Contention-free RA avoids collision by the eNodeB informing each UE which preambles may be used.
Referring to FIG. 6, the Physical Random Access Channel PRACH typically operates as follows (for contention based access):—
(i) As already mentioned the UE10 receives the downlink broadcast channel PBCH for the cell of interest (serving cell).
(ii) The network, represented in FIG. 6 by eNodeB 20, indicates cell specific information including the following:
                resources available for PRACH        preambles available (up to 64)        preambles corresponding to small and large message sizes.(iii) The UE selects a PRACH preamble according to those available for contention based access and the intended message size.(iv) The UE 10 transmits the PRACH preamble (also called “Message 1”, indicated by (1) in the Figure) on the uplink of the serving cell. The network (more particularly the eNodeB of the serving cell) receives Message 1 and estimates the transmission timing of the UE.(v) The UE 10 monitors a specified downlink channel for a response from the network (in other words from the eNodeB). In response to the UE's transmission of Message 1, the UE 10 receives a Random Access Response or RAR (“Message 2” indicated by (2) in FIG. 6) from the network. This contains an UL grant for transmission on PUSCH and a Timing Advance (TA) command for the UE to adjust its transmission timing.(vi) In response to receiving Message 2 from the network, the UE 10 transmits on PUSCH (“Message 3”, shown at (3) in the Figure) using the UL grant and TA information contained in Message 2.(vii) As indicated at (4), a contention resolution message may be sent from the network (in this case from eNodeB 20) in the event that the eNodeB 20 received the same preamble simultaneously from more than one UE, and more than one of these UEs transmitted Message 3.        
If the UE does not receive any response from the eNodeB, the UE selects a new preamble and sends a new transmission in a RACH subframe after a random back-off time.
As already mentioned, cells may be overlapping or even entirely contained within a larger cell. This is particularly the case for so-called Heterogeneous Networks.
FIG. 7 schematically illustrates part of a heterogeneous network in which a macro base station 10 covers a macro cell area MC, within which there are other, overlapping cells formed by a pico base station 12 (picocell PC) and various femto base stations 14 (forming femto cells FC). As shown a UE 20 may be in communication with one or more cells simultaneously, in this example with the macro cell MC and the picocell PC. The cells may not have the same bandwidth; typically, the macro cell will have a wider bandwidth than each pico/femto cell.
Some definitions are as follows:                Heterogeneous Network: A deployment that supports a mixture of more than one of macro, pico, femto stations and/or relays in the same spectrum.        Macro base station—conventional base stations that use dedicated backhaul and open to public access. Typical transmit power ˜43 dBm; antenna gain ˜12-15 dBi.        Pico base station—low power base station with dedicated backhaul connection and open to public access. Typical transmit power range from ˜23 dBm-30 dBm, 0-5 dBi antenna gain;        Femto base station—consumer-deployable base stations that utilize consumer's broadband connection as backhaul; femto base stations may have restricted association. Typical transmit power<23 dBm.        Relays—base stations using the same radio spectrum for backhaul and access. Similar power to a Pico base station.        
In LTE, an example of a femto base station is the so-called Home eNodeB or HeNB.
The installation by network customers of base stations with a localised network coverage cell, such as femto base stations (Home eNodeBs) is expected to become widespread in future LTE deployments. A femto base station or pico base station can be installed in, for example, a building within which network subscriber stations experience high path loss in transmissions with a macro cell. Femto and pico base stations can be installed by a customer in his own premises. The femto and picocells thereby formed can improve network coverage, but for coordination among the various cells, it is preferable for all the femto and picocells to be under the control of the macro cell (more precisely the MeNB 10 of FIG. 7), and synchronized with one another. When organized in this way, picocells can be regarded as transmission points of the base station, in addition to transmission points provided by the antenna ports of the base station itself.
The above-mentioned transmission points (TPs) include, for the purposes of the present disclosure both picocells and different sets of antennas within a macrocell at different geographical locations.
At present, a UE and an LTE network are only able to exchange information regarding the coverage of the network once the UE is at least connected to the network at the RRC level (although the UE may be in the RRC_IDLE state). A UE is not able to indicate its presence to the network ahead of an immediate need to transmit UL data, and the network must consistently maintain the transmission of certain broadcast signals (notably PSS/SSS and PBCH) whether they are useful to UEs in the vicinity or not. Such an architecture is not flexible to the changing UE distribution over time, and furthermore current network designs rely on a fixed association of transmission points (TPs) to cells. To provide dynamic deployment flexibility, efficient use of network transmit-power and to manage interference, it is necessary in future network architectures to allow the network to form or be configured dynamically around UEs as they move and as their service needs change over time.